Spatial Audio Raven Preamp

My (opinionated) tube preamp checklist:

* Must take full advantage of balanced DACs

* No Balanced to Single Ended input transformers

* Must support XLR balanced out and RCA single-ended out for use with balanced amps or single-ended amps.

* Balanced XLR out and single-ended RCA out must have equally good sound quality

* Not capacitor coupled

* Much prefer 6SN7s. Would consider 6SN7 family or DHTs. But balanced DHT preamp is large, heavy, pricey, with possible microphonics, etc.  

* No small signal tubes

* Stepped attenuators with a good remote. No potentiometers. No TVCs or AVCS. (I do like autoformers combined with transistor buffers. I think resistor attenuation is a better match with vacuum tubes)

* Must look good in living room. Prefer wood chassis. Conversation starter preferred.

* Tubes proudly displayed, not hidden in chassis

* Medium voltage gain

* Damper diode or solid state rectification. Damper diodes preferred. (I bought a bunch of 6AX4 damper diodes in 2015 in anticipation)

* State of the art power supply regulation

* Must be good value for money

More in next post...

Nice that the Raven ticks all the boxes! Seriously though, you’ve pinpointed (better than I could) what separates the Raven from a conventional Marantz 7C re-creation.

The preamps from the Fifties and Sixties followed a pattern of 2-stage 12AX7’s for lots of gain, a 12AX7 or 12AU7 cathode follower to knock down the output impedance, and plenty of feedback around the whole thing. Look at an Audio Research SP-3A or SP-6 and you see the same circuit. This is what most audiophiles think is "tube sound" for the simple reason that hundreds of thousands of preamps and integrated amps were built just this way, so it’s a very familiar sound.

But there are other ways to build a preamp, borrowing from studio electronics going back to the 1930’s. That’s the Raven, with no coupling caps, zero feedback, and a fully balanced circuit all the way through.

More technically, when the Marantz (or similar) circuit is compared to the Raven, there are 1, 2, or 3 coupling caps in the signal path. "Tuning" the sound of a classical preamp is usually little more than a cap swap, leaving the circuit itself untouched. The classical preamp is single-ended, relying solely on feedback to get distortion down to acceptable levels. And the sonics of cathode followers remain controversial, depending on the nature of the cathode load (resistive, inductive, active current source, etc.).

Lynn,

Yeah, no negative feedback and single stage tube design should be added to my checklist.

I recall first reading your articles on your nutshellhifi.com site -- "The Amity, Raven, and Aurora" and "The Karna Amplifer" close to 20 years ago.

The schematics made a big impression on me back then - quite foreign but so inventive - a modern take on an old Western Electric brain trust concept.

After finding this Raven thread recently, I then found the related Audiogon thread "300b lovers":

https://forum.audiogon.com/discussions/300b-lovers?highlight=300b%2Blovers

which I thought in some ways was as significant for tube amplifier design as your "Beyond The Ariel" discussion on diyaudio.com was for speaker design.

Anyone interested in the Raven preamp or Blackbird monos and also interested in amp design might consider reading the 300b lovers thread carefully.

After my own dilettante experience building low WAF tube preamps, owning various commercial tube, transistor, and passive preamps, and my search for an appropriate commercially produced tube preamp, then reading these two Audiogon threads, and recalling your Nutshellhifi writings, it really came together for me that I’d be able to own a pretty flexible natural sounding balanced preamp.

@williamdc

Glad you are enjoying your new Raven.  It does seem to tick all of your boxes!  We arrived there after much experimentation and listening.   I have always loved damper diodes, so I knew we were starting there.  The transformers took a few rounds to get exactly the performance we sought.  I had used the Khozmo attenuators for years, but I had to think hard about how to get them to do what we wanted.  It all came together.  You will find the bass improves over the first 100 hours as the very large cathode bypass film caps run in.  You get most of the sonics after 10-20 hours, but it subtly improves.  Enjoy!

Links to the aforementioned articles:

The Amity, Raven, and Karna Amplifiers

These articles date back to 2004 or earlier, and some of my opinions have changed since then. The availability of custom transformers opened a broader palette of tube selections, and Don’s work on precision power supplies offered a wider range of possibilities in the overall design.

I find it interesting that my objections to the sound of coupling caps is as strong now as it was back in 1997. Once you accustom to the sound of no coupling caps, adding them back in the signal chain is very noticeable. Much of what we expect of "tube sound" is nothing more than the sound of XYZ capacitors.